1. Deficiency And Excess Of Development

A total absence of the spinal cord, such as occurs when both the head and the trunk are wanting, is rarely met with: but a partial deficiency of it is more frequently observed. In cases of acephalus the defect is proportioned to the extent to which the neck, thorax, and even the abdomen are wanting. Allied to this is another partial deficiency, in which parts of the cord are arrested in their development in bulk: thus, when the upper or lower extremities are wanting, or withered, there is no enlargement in the cervical or lumbar regions of the cord.

An original deficiency, whether complete or partial, may be distinguished from that which, being produced by the destruction of the foetal medulla, merely has the semblance of being original, in just the same manner as hemicephalus and acephalus are distinguished from each other (p. 280). Hydrorachis is, I believe, as much known by the name of spina bifida, when it is a disease of the spinal cord, as when, in the form of foetal dropsy, it affects only the spinal arachnoid sac (p. 252), - provided only the vertebral column be fissured. And I believe it will be best to treat of that disease in this place, referring to what has been said at p. 176, for the condition of the vertebral column.

In its highest degree, the cord is altogether wanting, and the nerves terminate on the inner side of the arachnoid. The vertebral canal is then usually wide open posteriorly - fissured, that is, to a great extent; the spinal membranes, especially the dura mater and arachnoid, are intimately united with one another, and shreds of pia mater hang within the latter from the extremities of the nerves. The membranes, like the bony canal, are open posteriorly; and the general integuments, deficient to the same extent, adhere to them on both sides at the margin of the fissure. Not unfrequently a streak of hair extends along the margin of the skin from the head downward, and just overhangs the line of adhesion. The posterior surfaces of the bodies of the vertebra are thus laid bare, being covered merely by the spinal membranes.

Sometimes, as an exception to this arrangement, the membranes of the cord are not open, but form a closed bag filled with serum. In some very rare cases of this kind the vertebral column also is not fissured.

In a less advanced degree of the disease, rudiments of the cord remain, and especially its anterior columns, more or less complete.

In a still less degree, the spinal cord is fissured, and appears open behind; it is broad and flat. The membranes and the vertebral canal are in the condition mentioned above.

Allied to this is an extremely rare case, in which the canal of the spinal cord still remains, and is dilated in consequence of chronic dropsy within it.

The degrees just spoken of are measured by the amount of destruction of the cord, and by the arrest in its complete development, without reference to their local extent. Each degree may affect the whole length of the cord, or only portions of it; and the simultaneous fissure in the vertebral column may constitute accordingly either a total or a partial (local) spina bifida. The latter may exist at any portion of the spine, or at two different parts in the same subject; the spinal membranes are then either fissured in the manner above described, or they are merely distended, and form the well-known membranous sacs in the spinal region described at page 252.

Every case of hydrorachis is, usually at least, combined with hydrocephalus; but when the whole spine, or its cervical region, is fissured, the kindred disease, hemicephalus, or else hydrencephalocele, is always present too.

Hydrorachis is similar in its nature to congenital hydrocephalus; and when it reaches advanced degrees, it produces spina bifida, just as hydrocephalus occasions hemicephalus and hydrencephalocele.

The consequences of hydrorachis, are self-evident, when reference is made to what has been already said on the subject of dropsical arachnoid sacs.

Spina bifida in its slightest degree is occasioned by dropsy of the arachnoid sac; the cord maintains its integrity, but the development and closure of the arches of the vertebrae are arrested, and the arachnoid bulges through the apertures in the posterior wall of the vertebral canal: it is generally only partially dilated, and forms a sac, such as has been already described (p. 252). The chief situation of this form of spina bifida is the lumbar, lower dorsal, and sacral regions.

In a few cases the cord itself passes through the fissure in the vertebrae, and a hernia of the spinal cord takes place.

Excessive development of the spinal marrow is met with only in double monsters. Its form varies according to the extent of duplication, etc.

2. Anomalies In Size

The length of the spinal cord generally has some relation to that of the vertebral column: so that, for instance, in individuals in whom a tail exists, the cord is unusually long. But sometimes there is more or less of disproportion between the two: the cord is unusually short; or being unusually long, it suggests the recollection of its condition in the foetus. Thus, for example, in fission of the vertebrae (spina bifida) it is in part of really unnatural length, and partly it has the appearance of being too long, because the vertebrae are arrested in their development, and frequently at the same time malformed.

The thickness of the spinal cord is subject to many peculiarities in different individuals, as well as to various local anomalies, which affect particularly its enlarged portions, and depend on deficient or excessive development at the periphery of the nervous system.

Considerable morbid enlargements in the bulk or diameter of the cord result from various anomalies. Some of the enlargements are local, and are bounded by a more or less sharp margin; while others, on the contrary, extend throughout the whole cord. The principal are congestion, apoplexy, oedema, inflammation, serous effusions into a persistent foetal canal, similar effusions which fill up the space when the gray tract is atrophied, morbid growths, and so on, and lastly hypertrophy. The last is the only one which properly belongs to the present section.